1. Field of the Invention
This invention relates to sensors for detecting magnetic fields, and more particularly, to a magnetoresistive sensor utilizing a metallic compound which undergoes a electronic phase transition in a magnetic field form an antiferromagnetic state to a ferromagnetic state and vice versa when the magnetic field is removed and has a corresponding "giant magnetoresistive effect".
2. Description of the Prior Art
Magnetoresistive devices exhibit a change of resistance to current passing therethrough when the magnetic field is changed. Typically, two layers of ferromagnetic material are used to form a magnetoresistive device and wherein the field intercepted by the magnetoresistive device exceeds the coercivity of one of the layers in order to provide a change in direction of the magnetization in one of the layers. Magnetoresistive devices have been used for detecting magnetic fields from magnetic tape and direct access storage devices (DASD). For example, a magnetoresistive device is positioned in a head attached to a slider in a DASD. The head is positioned over a magnetic layer on a rotating disk and detects magnetic fields emanating from the magnetic layer indicative of data stored therein. Examples of magnetoresistive devices are described in U.S. Pat. No. 4,476,454 which issued on Oct. 9, 1984 to J. A. Aboaf et al. which describes devices and circuits employing magnetoresistive materials exhibiting a negative (.DELTA..rho.) change in resistivity effect. A more recent magnetoresistive device showing a .DELTA..rho.=9 with a magnetic field in the range from 20 to 120 Oe is described in U.S. Pat. No. 5,159,513 which issued on Oct. 27, 1992 to B. Dieny et al. entitled, "Magnetoresistive Sensor Based on the Spin Valve Effect".
While a change of 9% resistivity may seem large, there is a need in the industry to have magnetoresistive devices which exhibit changes in resistivity greater than 9% at a lower magnetic field.
Some magnetoresistive sensors have utilized an antiferromagnetic layer to provide a longitudinal exchange bias in a ferromagnetic layer. The exchange bias merely fixes the direction of magnetization in the ferromagnetic layer. One such device, is described in U.S. Pat. No. 5,014,147 which issued on May 7, 1991 to S. P. Parkin et al. which uses an alloy from Fe(.sub.1-x) Mn.sub.x where x is within the range of 0.3 to 0.4, as an antiferromagnetic layer.
Certain metallic compounds have been known to undergo a magnetic phase transition from antiferromagnetic (AF) to ferromagnetic (FM) that are found in nature as well as fabricated or formulated in a laboratory. One such metallic compound is iron rhodium (FeRh). Iron rhodium undergoes a phase transition at about 340 K. from antiferromagnetic to ferromagnetic. The phase transition is accompanied by an abrupt increase in the specific volume of iron rhodium. It has been published that the transition temperature of iron rhodium can be lowered by applying a magnetic field H.sub.cr and that the transition temperature can be raised by applying a positive pressure P.sub.cr. Experiments by J. S. Kouvel and C. C. Harteluis, J. Appl. Phy., Supp. to Vol. 33, 1343 (1962) on a crude polycrystalline sample of FeRh showed a 30 percent drop in resistivity on heating through the magnetic phase transition.
The use of iron-rhodium films and alloys thereof for magnetic recording was described in U.S. Pat. No. 3,607,460 which issued on Sep. 21, 1971 To J. M. Lommel. In '460, an electron beam heated individual regions through a first order transition to the ferromagnetic state whereupon the regions are permitted to cool to a biasing temperature slightly higher than the temperature of the transition back to an antiferromagnetic state. A magnetic field was then applied to the entire film to magnetize only those regions of the film in the ferromagnetic state and read out of the recorded information was achieved by conventional electron beam microscopy. The ferromagnetism of the film could be erased by cooling the film below the transition temperature to the antiferromagnetic state or by the application of a strain to the film.